Positron emission tomography (PET) scanning employs a gamma-emitting radiopharmaceutical ingested by a patient or injected into a patient. Multiple gamma ray images are taken in multiple directions to generate a 3-dimensional PET image and/or multiple slices of a PET image. PET scanning generally provides useful information regarding the functional condition of the body tissues and systems such as the cardiovascular system, respiratory system, and/or other systems. PET scanning is useful for indicating the presence of soft tissue tumors or decreased blood flow to certain organs or areas of the body. During operating, image quality of a PET scan can be affected by motion during imaging, for example, respiratory and/or cardiovascular motion. Imaging artifacts may be generated during acquisition because of body motion. PET scans can require a relatively long duration data acquisition period, on the order of several minutes (e.g., about 30 minutes per image) for a typical clinically sufficient image. Typically, a large number of PET data acquisitions (e.g., frames) are acquired at multiple bed positions during the imaging period. Consequently, patient movement is a problem in PET scanning.
PET scanning has a limited field of view (FOV) and cannot capture whole body images. In conventional systems, in order to perform whole body imaging, multiple PET images are captured at multiple positions with respect to a patient (e.g., beds). The multiple images are obtained by a “step and shoot” method. FIG. 1 illustrates one embodiment of a step and shoot PET scan. In the step and shoot method, the PET scanner 100 is positioned at multiple, discrete bed locations and an image is obtained at each bed. The counts of each line of response (LOR) 104a, 104b in the obtained sinogram are an integration of signals over a single crystal pair 106a, 106b. Different LORs 104a, 104b see the same gating windows. The multiple images are stitched together to form a single, whole body image. A gating profile may be used to reduce image blur because of movement.
Recent systems have employed a continuous bed motion (CBM) method to obtain whole body images. FIG. 1 further illustrates one embodiment of a CBM PET scan. In CBM systems, the PET scanner 100 is moved from a start position, for example, at the head of a patient, to an end position, for example, the feet of a patient, at a constant rate. PET data is collected continuously from the start position to the end position. CBM data acquisition is more natural for patient whole body scan and provides more flexibility in data processing. However, CBM data acquisition complicates normalization for cardiac/respiratory gating protocols, which are commonly used to reduce motion artifacts in PET scans. In CBM scanning, counts are established for virtual LORs 110a, 110b. The count for each virtual LOR is an integration of multiple real LORs 104a, 104b. Each virtual LOR 110a, 110b receives contributions from different gating windows, complicating normalization.
If gating effects are not accounted for properly, image non-uniformity and wrong quantification will occur. For step and shoot scans, counts acquired on LORs can be regarded as integrations of counts on the same crystal pair over time. Gating windows will act on LORs of all sinogram planes simultaneously. In CBM scans, data corresponding to virtual LORs 110a, 110b of a sinogram chunk are integrations of counts acquired by spatially distant detector pairs. Different virtual LORs 110a, 110b in a sinogram chunk will have contributions from counts detected in different sets of gating windows. Simple scaling methods, as used in step and shoot scans, do not properly account for gating effects on reconstructions of CBM sinograms.